Project. 2. NK-1R antagonists, including aprepitant and others, may interrupt NK-1R and CCR5 crosstalk at several different levels including receptor: receptor interaction, amplification of signal transduction, regulation of receptor synthesis and expression, or indirectly through effects on cytokine-chemokines (CCR-5 ligands). We propose that SP signaling through NK-1R has a selective regulatory role in the regulation of the chemokine receptor CCR5 in monocyte/macrophages. CCR5 and NK-1R are G-protein coupled receptors (GPCRs). We hypothesize that activation of the signaling pathway by the SP:NK-1R autocrine loop results in amplification of CCR5 mediated signaling. Further, the interruption of this autocrine loop by NK-1R antagonists, results in functional changes in the CCR5 receptor resulting in inhibition of HIV infection and replication. The overall goal of this project is to determine whether SP: NK-1R interaction regulates CCR5 function through a) GPCR crosstalk at the level of the receptor or through crosstalk between signaling pathways, b) a direct effect on CCR5 expression through alterations in receptor synthesis, or c) indirectly through alterations in cytokine and chemokine synthesis and release. Specific Aim 1: Will characterize the antiviral and immunomodulating effects of aprepitant and other candidate NK-1R antagonists on CCR5 mediated cellular functions in monocyte/macrophages. Specific Aim 2: Test the hypothesis that NK-1R antagonists alter CCR5 mediated physiological responses through G-protein coupled receptor (GPCR) crosstalk. Specific Aim 3: Test the hypothesis that NK-1R antagonists interrupt synergistic crosstalk between NK-1R and CCR5 signal transduction pathways. Specific Aim 4: Further define the specificity of aprepitant and other NK-1R antagonists for the NK-1R receptor through knockout studies using siRNA to deplete NK-1R receptor expression in the macrophage cell line THP-1. These studies will determine the specificity/selectivity of NK-1R antagonists in monocyte/macrophages and guide selection of the optimal antagonist for HIV therapeutic development.